Methods, systems and apparatus for modeling capacitance for a structure comprising a pair of long conductors surrounded by a dielectric material and supported by a substrate. In particular, the structure may be on-chip coplanar transmission lines over a conductive substrate operated at very high frequencies, such that the substrate behaves as a perfect dielectric. It is assumed that the surrounding dielectric material is a first dielectric with a first permittivity (e.sub.1) and the substrate is a second dielectric with a second permittivity (e.sub.2). A method models the capacitance (C.sub.1) for values of the first and second permittivity (e.sub.1, e.sub.2) based on known capacitance (C.sub.2) computed for a basis structure with the same first permittivity (e.sub.1) and a different second permittivity (e.sub.2). Extrapolation or interpolation formulae are suggested to model the sought capacitance (C.sub.1) through one or more known capacitances (C.sub.2).